When two wireless devices communicate with each other in a radio network in a traditional manner, each wireless device communicates radio signals with a serving base station of the radio network by sending uplink radio signals to the base station as well as receiving downlink radio signals from the base station. This is the traditional way of communication in a radio network also when the two wireless devices are located somewhat close to one another and being served by the same base station. This transmission mode will be referred to as a cellular mode of communication. Recently, techniques have been developed to enable such wireless devices in a radio network to communicate radio signals with each other directly, as controlled by the radio network and using frequency spectrum licensed to the network, such that each wireless device receives and decodes the actual radio signals that are transmitted from the opposite, or “peer”, wireless device. Bluetooth is another example of direct communication between wireless devices, although without control or involvement by any network and using unlicensed frequency spectrum.
Communication of radio signals may thus take place directly between the two wireless devices without the radio signals being communicated over the radio network via one or more base stations, which is a transmission mode that will be referred to as a direct mode of communication. In that case, the serving base station allocates radio resources, e.g. defined by time and/or frequency, which the wireless devices are allowed to use in the direct communication. Such direct radio communication between two wireless devices is commonly referred to as “Device-to-Device, D2D, communication” or terminal-to-terminal communication.
In the field of cellular radio technology, the term “wireless device” is usually used and will be used in this disclosure to represent any wireless communication entity capable of radio communication with a cellular radio network including receiving and sending radio signals. Another common term in this field is “User Equipment, UE” which is often used for various wireless devices such as e.g. mobile telephones, tablets and laptop computers. However, a wireless device in this context is not necessarily operated by a human user. It could also be a machine-to-machine type of device operating automatically such as a sensor, counter or measuring entity.
Further, the term “base station”, sometimes also referred to as a network node, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc., represents any node of a cellular radio network that is arranged to communicate radio signals with wireless devices. The base station described here may, without limitation, be a so-called macro base station or a low power base station such as a micro, pico, femto, Wifi or relay node, to mention some customary examples. Throughout this disclosure, the terms “network node” and “User Equipment, UE” can further be used instead of base station and wireless device, respectively.
The above D2D communication may thus be employed whenever the two wireless devices, also referred to as “peer devices” or just “peers”, are close enough to one another to be able to receive and decode direct radio signals from the opposite peer. Thereby, it may be possible to reduce transmit power in the area and also to reduce interference, as compared to what is required to enable a serving base station to communicate radio signals with the wireless devices in the traditional manner.
In a conventional cellular communication between a base station and a wireless device, a radio signal transmitted by the wireless device may be successfully received and decoded by the base station provided that the current radio conditions allow for sufficient quality of the received signals. This means that the received signal should not be too weak and/or interfered too much by other radio transmissions in the neighborhood for satisfactory reception and decoding. For example, the wireless device may be situated close to the cell edge and relatively far from the base station, or in a spot with bad radio coverage, so that the radio signal fades considerably on its way to the base station. Furthermore, the wireless device may in that case need to transmit with increased power in order to provide a sufficiently strong signal at the receiving base station, which may cause interference to other nearby communications. Another possibility is to add redundant bits which can be used to assist the decoding in the base station's receiver although they occupy precious radio resources such that overall data throughput is reduced.
In a similar manner, when the direct mode is employed between two wireless devices, a radio signal transmitted by one wireless device may be successfully received and decoded by the other wireless device provided that the current radio conditions allow for sufficient quality of the received signals. The requirements for successful reception and decoding are thus similar to the case of cellular mode discussed above. It is also known to use a third wireless device as a D2D relay between the two communicating wireless devices where the third wireless device is in a fixed position and utilized as a part of the network infrastructure. However, a problem with such relays is that deployment is costly and the fixed position of the relay may make it unsuitable for communication, e.g. when one or both of the two communicating wireless is/are positioned too far away from the relay for favorable signal reception.
It is thus a problem to achieve sufficient signal quality over a D2D radio link between two wireless devices or over respective cellular radio links between the wireless devices and a base station, without causing too much interference and/or reduction of data throughput, particularly under less than optimal radio conditions.